Whole Foods shares tumbled as investors lost faith in the grocer's command on the organic food market

Whole Foods stock plummeted nearly 20 percent Wednesday, its biggest one-day fall in more than five years, after informing skittish investors late on Tuesday that sales at established stores have slowed.

The grocery chain has been fighting to compete in an increasingly crowded natural and organic foods market, a niche it once had cornered.

“It’s just gotten to be a very big niche and in some ways, it’s gone mainstream,” John Mackey, chief executive, told analysts on a conference call, the Financial Timesreports.

Whole Foods’ second-quarter net income was $142 million, flat compared to a year earlier. But it predicts same-store sales growth of 5% to 5.5%, versus a previous forecast of 5.5% to 6.2% and historical growth of 7% to 8%, the Wall StreetJournal reports. The company’s competitive edge traditionally derived from its low prices, but other major companies are increasingly focusing on selling low-cost organic product lines as consumers pay closer attention to the foods they eat.

Walmart’s U.S. division plans to introduce 100 Wild Oats-branded organic products ranging from olive oil and canned black beans, Reuters reports. Walmart said those products would be priced at least 25 percent below branded organic foods. With that kind of competition, Wall Street may not have an appetite for Whole Foods for long.